Due to the trend of high-speed digital systems attributed to technological developments, Electro-Magnetic Interference (EMI) has emerged as a challenging issue to tackle. To mitigate EMI, an EMI filter was used or EMI shielding was applied in an initial stage. However, the use of an EMI filer or the EMI shielding is not cost-effective and has technical limitations. That's why a Spread Spectrum Clock Generator (SSCG) has been widespread recently. For EMI reduction, the SSCG modulates a reference signal with a high energy in a specific frequency to a signal with a low energy relative to the reference signal in a frequency having a predetermined bandwidth. As a clock signal is spread across a specific frequency band by the SSCG, rather than it is fixed to a specific frequency, the energy of the clock signal in the specific frequency is distributed and thus the clock signal does not cause EMI to adjacent electronic circuits.
FIG. 1 is a block diagram of an SSCG that includes a pre-divider 10, a phase frequency detector 12, a charge pump 14, a loop filter 16, a combiner 18, a Voltage Controlled Oscillator (VCO) 20, a modulation controller 22, a Multiplexer (MUX) 24, and a feedback divider 26. The phase frequency detector 12, the charge pump 14, the loop filter 16, the VCO 20, and the feedback divider 26 collectively form a typical Phase Locked Loop (PLL).
The SSCG illustrated in FIG. 1 has blocks used for modulation in the PLL in order to carry out a spread spectrum function. The spectrum spreading can be achieved largely in the following four methods. One method is a fine control of the feedback divider 26 through the modulation controller 22. A second spectrum spreading method is to apply a modulation voltage directly to the loop filter 16 through the modulation controller 22. The modulation voltage is combined with the output of the loop filter 16 in the combiner 18. A third spectrum spreading method is to provide modulation information to the charge pump 14 through the modulation controller 22. A fourth spectrum spreading method is to control the output phase of the VCO 20 through the modulation controller 22 and the MUX 24. The modulation controller 22 generates a selection signal for the MUX 24.
The SSCG using the foregoing methods should transmit a typical modulation frequency ranging from tens of kHz to hundreds of kHz without distortion. The bandwidth of the PLL should be set to be low, such as about a tenth of the modulation frequency, e.g., ranging from several kHz to tens of kHz.
As the modulation frequency gets lower, the PLL bandwidth should be decreased. As a result, the capacitance of the loop filter 16 should be increased to or above several nF. It is difficult to integrate such a capacitor having a capacitance of several nF on a chip. For this reason, most spread spectrum PLLs adopt external filters. The use of additional parts increases cost.